Control system for pulverizer mills



Aug. 2091940. 1'. A. PEEBLES 7 CONTROL SYSTEM FOR PULVERIZER MILLS FiledNov. 27, 1936 2 sheets-Sheet 1 2 8 I: a /17 i 2e 5 INVENTOR Y E1 7 20,1940- T. A. PEEBLES 2,212,125

I CONTROL SYSTEM FOR PULVERIZER MILLS Filed Nov. 27, 1956 2 Sheets-Sheet2 Bai/er 5 HIE, Sfeam Header- Patented Aug. 20, 1940 PATENT OFFICE2,212,125 con'mor. SYSTEM son PULVERIZER -MILLS Thomas A. Peebles,Pittsburgh, Pa.,

John' M. Hopwood, Mt. Lebanon,

assignor to Application November 27, 1936, Serial No. 112,935

9v Claims.

This'invention relates to pulverizer mills, such as are used, forexample, to pulverize coal for boiler furnaces and more particularly toa control system for regulating the rate of feed of raw coal to thepulverizer mill primarily in accordance with the demand for pulverizedfuel and secondarily, controlling the feed in accordance withthe load onthe prime mover that drives the mill.

An object of this invention is. to provide a control system forpulverizermills that shall be capable of changing the rate of feed. ofmaterial to the mills to bepulverized, either automatically or by hand,in accordance with the demand for pulverized material, and of somodifying the rate of feed of unpulverized material that a balance ismaintained between the rate of feed to and the amount of pulverizedmaterial delivered by the mill for any given rate of demand therefor.

Another object of. the invention is to provide a control system thatshall be automatic in operation, for example in connection with a steamboiler plant, and adaptable to pulverizer mills that are driven eitherby steam engines or turbines or electric motors.

The above stated objects and other objects of the invention will. beapparent to those skilled in this art, from the following descriptiontaken in conjunction with the accompanying drawings, in which: Figure 1is a more or less diagrammatic view of a pulverizer mill provided with acontrol system arranged and constructed in accordance with an embodimentof the invention;

Fig. 2 is a'view similar to Fig. 1 showing a modified form of controlsystem;

Fig. 3 is a view of a control system of the type shown in Figs. 1 and 2but modified for application to pulverized millsdriven by electricmotors,

m the mills of Figs. 1 and 2 being driven by steam turbines; I

Fig. 4 is a sectional view of a regulator embodied in the control systemof Fig. 3. and adapted for use where the pulverized mill is driven by anelectric motor.

Throughout the drawings and the specifica-v tion, like referencecharacters indicate like parts. I

The invention herein disclosed is adapted for use with pulverizer mills,that is mills that grind coarse material to such a degree of finenessthat the pulverized material may be carried away to the point of use bya carrier such as air, for example. Speciflcally, the system is shownand described in connection with a mill employed to pulverize coal to beconveyed by air to the burners of a furnace, for example a boilerfurnace.

In the operation of a pulverizer mill, for furnace operation, the coalat a rate corresponding to the rate at which it is burned, and that thecoal be fed to the pulverizer mill at a rate corresponding to the rateatwhich it is ground in the mill and carried therefrom by an air blastto the burners. If coal is fed too fast to the mill, the mill grindsmore coal than is necessary for a given demand,

or if the coal is fed too slowly, not enough coal is ground to supplythe demand. The rate of feed of coal to the mill is influenced also bythe condition of the coal; for example, the state of dryness affects therate of feed, in that wet coal does not feed as fast as dry coal.

In Fig. l of the drawings, a control system embodying one form of theinvention is illustrated as applied to a pulverizer mill l driven by ,a.prime mover 2 indicated diagrammatically as a steam turbine. Coal is fedto the pulverizer from a hopper 3 by means of a feeder 4 driven by anadjustable-speed electric motor 5. The speed of the electric motor iscontrolled by a rheostat 6 connected in circuit with the field windingof the motor, and by adjusting this rheostat, the rate of feed to thepulverizer may be varied to meet varying demands for pulverized fuel.Thisadjustment of the rheostat may be accomplished by hand orautomatically by means of a regulator 6', disposed to operate inresponse to an operative that reflects the demands for combustion. Inthe case of a steamboiler, variations in pressure of the steam indicateeither that more or less coal must be burned to maintain the steampressure at the desired value. vlj or that the load is increasing andthat the combustion rate is insuflicient to supply the demand for steam,or if the pressure .is increasing, that indicates that the load andtherefore the demand for steam is falling ofi and that the rate ofcombustion should be reduced. In, the one case, the rate of delivery 'ofpulverized fuel to the burners must be increased which means that thepulverizer must grind more coal per unit of time and in order to do thatthe rate of feed must be increased.- In the other case, the rate ofdelivery of fuel to the burners must be decreased, requiring therefore,a decreased rate of feed of coal to the pulverizer.

The coal as it is pulverized is carriedaway by a stream of air passingthrough the pulverizer,

it is desirable that the mill grind 5 to the burners of the furnace. Theair supply is also regulated in accordance with the rate at which thecoal is pulverized, all as is well known in this art.

The air supply to the pulverizer may be controlled by -a regulator I andthis regulator may be adjusted by hand or it may be operatedautomatically in response to changes in the demands for combustion.

If the condition of the coal to be pulverized always remained fixed andconstant, the feed of coal to the pulverizer as controlled by rheost'at6 would be sufficient to meet the. demands. But in practical operationthe condition of the coal varies so that, insome cases, for a.givendemand for powdered coal, the coal would be delivered to the pulverizerat a rate higher than required. At other times, not sufiicient coalwould be delivered to meet the demand.

The load imposed on the turbine which drives the pulverizer indicatesthe amount of coal that is being pulverized. Thus for example, if theturbine operates at variable loads, it will be apparent that thepressure of the steam as delivered to the nozzles will vary with thedegree of opening of the governor controlled throttle valve 8.

In order to obtain a proper balance between the demand for pulverizedfueland the rate of feed of fuel to the pulverizer, aregulator 9 isprovided which responds to a control impulse delivered thereto by amaster regulator M. R. in response to variations in the demand for steamon the boiler and to the pressure of the steam at the nozzles of theturbine. I

Regulator 9 comprises a regulating element, for example, an escapementvalve II) that is actuated jointly by a pressure responsive element iiand a pressure responsive element l2 both acting on a lever or beam l3.Beam l3 carries a stem l4 positioned to move the escapement valve Ieither toward an inlet port l5 and away from an exhaust port it or viceversa, depending upon the relative magnitudes of the impulses or forcesacting on elements II and I2.

Beam I3 is fulcrumed at one end on a knife edge I! and connected at theother end by means of a tension spring i8 to element ll illustrated ascomprising a bellows. If bellows ll moves upwardly in response to a.decreasing impulse delivered to a housing I 9 within which it isdisposed, spring l8 tends to elongate and move beam l3 upwardly,shifting the valve I 0 towards its inlet port and away from its exhaustport. When moving in this direction the pressure within valve body 20and which is transmitted to a sending line 2|, to regulators 6' and 1,is also reduced. The minimum pressure occurs in line 2| when inlet porti5 is closed and the exhaust port I6 is open. If thebeam is moveddownwardly either as the result of increasing pressure in housing I901'' of an increasing pressure acting on element l2 disposed within ahousing 22, the valve moves towards exhaust port 16 and away from inletport' I5 whereby the pressure within the valve body.

header 23. The steam pressure acts on a pressure.

responsive .element, such as a bellows 24 disposed within a pressuretight chamber 25 and this bellows is connected by a stem or push rod 26to a lever 21 fulcruined between knife edges 28 atone of this leveractuates an escapement valve 30 like the valve Hi just described inconnection with regulator 9.

When the steam pressure rises, bellows 24 is compressed and moves thepush rod upwardly against the tension of spring 29 thereby causing theescapement valve to move away from its inlet port and to allow anincreased pressure to be transmitted to the housing in which the bellowsIi is disposed. 'If the steam pressure is failing, the bellows expandswhereby the valve is moved towards its inlet port and away from theexhaust port thereby decreasing the pressure imposed on bellows II. Inother words, with increasing load on the boiler, the pressure acting onbellows Ii decreases, and with decreasing load on the boiler thepressure on this bellows increases.

In order to stabilize regulator 9, a dash-pot 34 is provided that actson beam l3 in such manner as to avoid excessive and sudden movements.Justments and bring matters under their control to somewhere near theproper point before requiring further adjustment if any is necessary.Lever i3 is connected by a yoke 3| to bellows l2 and the yoke isconnected by means of a spring 32 to the movable element or piston 33 ofthe dashpot 34. The dash-pot comprises a cylindrical member 35 having areservoir 36 at its upper end and a variable volume chamber 31communicating with the lower end of the cylinder. Chamber 3'! is definedby a bellows 38 which is urged towards its maximum volume position bymeans of a spring 38'. This bellows is disposed within a chamber orhousing 39 to which pressure impulses are delivered by the escapementvalve ID. If the pressures delivered to housing 39 are increasing,bellows 38 is compressed whereby fluid is displaced causing piston 33 tomove upwardly thereby compressing spring 32 and yieldingly opposingdownward movement of lever l3. A needle valve ll associated with abypass 42 which is connected at its lower end to cylinder 35 at a pointbelow piston 33 and at its upper end to the reservoir at a point abovethe piston, allows fluid to escape from the space below the piston tothe space above the piston. The adjustment of this needle valve therebydetermines the length of time the dash-pot is effective in checkingmovement of lever i3. As soon as the liquid has been transferred to thereservoir above the piston, the eii'ect of the dash-pot is nullified. Ifthe pressure delivered to housing 39 is decreasing, the bellows tends toexpand and increase the volume of chamber 3'! so that the tendency is topull the dashpot piston downwardly. The expansion of the bellows allowsfluid to flow from the reservoir above the piston through the needlevalve into the space below the piston and the efl'ect oi the dash-pot onthe regulator is therefore gradually and finally nullified.

In any case, whether the pressures delivered to housing 39 areincreasing or decreasing, the

. eflectiveness of the dash-pot is made proportional to thepressuredelivered to it.

Assuming that the boiler is operating at a point where sufiicient fuelis supplied to maintain the steam pressure in the steam header at thedesired value, and that an increased demand for steam takes place, thenthe steam pressure will be reduced whereby regulator MfR. reduces themitted to bellows I2.

pressure imposed on bellows II. This causes the bellows to expand andincrease the tension in spring I8 whereby the left hand end of beam I3is moved upwardly, causing valve III to move towards its inlet port andreduce the pressure delivered to the dash-pot chamber 39 and to thesending line 2I. This reduced pressure in the sending'line iscommunicated to regulators 8' and I. In response to this reducedpressure impulse, regulator 6' adjusts rheostat 6 in a direction toincrease the speed of motor 5 and the rate of feed of coal to thepulverizer mill, and regulator 1 moves in a. direction to open a damper43 to increase the amount of air delivered to the pulverizer mill.

The increased rate of feed of coal to the pulverizer mill imposesgreater load on turbine 2. This tends to decrease the speed of theturbine causing the governor operated valve 8 in the steam line tothe'turbine to open wider. Asthis governor operated valve opens thesteam pressure at the steam nozzles (not shown) of theturbine increasesand this increased pressure is communicated to housing 22 within whichbellows I2 is disposed causing this bellows to be compressed and topulldownwardly on lever I3 thereby tending to-move valve III away fromits inlet port. This increases the pressure impulses delivered to therheostat operating regulator 6' and the air control regulator I. 7

When the forces imposed by bellows II an I2 on lever I3 are inequilibrium, the lever comes to rest but this results in such a ie-adjustment of the positions of the movable elements of. regulators 6'and I that the' rate of feed of fuel to the pulverizer is caused tocorrespond to the rate at which it is delivered from the pulverizer tothe furnace. If for some reason the rate of feed to the pulverizershould increase without any change in demand for steam or without anychange in loading pressure on bellows I I, the load imposed on turbine 2would increase whereby an increased steam pressure would be. trans- Thiswould disturb the equilibrium between bellows I2 and II and cause valveIll to transmit increased impulses to regulators 6' and 1, therebyreducing the rate of feed of coal to the pulverizer and the rate of airdelivery thereto to a value corresponding to that determined by thedemand for coal on the pulver-.

izerl When this condition is attained the bellows II and I2 are inequilibrium. On the other hand,

should the rate of feed decrease the load on the turbine would decreasethereby causing the steam pressure acting on bellows I2 to decrease, andunbalance the forces exerted by bellows II and I2 on beam I3. Such anunbalance results in regulator 9 sending out impulses to regulators 6'and I that would cause the one to increase the rate of fuel feed and theother to open damper 43 and increase the rate of air supply to thepulverizer. As soon as balance has been restored between bellows II andI2 the rate of feed of coal to the pulverizer mill will remain fixeduntil the master regulator changes the loading pressure on bellows II,in response to a change in steam demand. d

From the above it is seen that the rateof fuel feed to the pulverizer iscaused to bear such a relation to the demand for pulverized fuel thatthe feed of fuelto the pulverizer is caused to bear a fixed orsubstantially fixed relation to the amount of pulverized fuel deliveredby the pulverizer to the furnace. Any change in the duty imposed on thepulverizer is primarily controlled by the master regulator actingthrough regulator 5' and secondarily by the unbalance between theloading pressure sent from master regulator M. R. to regulator 9 and thesteam pressure at the nozzles of the steam turbine acting on bellows I2.

In Fig. 2 of the drawings, regulator 9, indicated only schematically,and regulators 6' and l which control the feed of fuel to the pulverizerand the air pulled through the pulverizer are directly responsive toimpulses sent out from the master regulator M. R. Feed control regulator6' is also controlled by regulator 9. In the system shown in Fig. 2steam turbine 2 drives the pulverizer and a suction fan 46 pulls airthrough the pulverizer, the air carrying with it pulverized coal throughduct 41 into the fan from which it is discharged through a pipe 48 tothe furnace (not shown) Regulator 6' operating rheostat 6 is controlledby a dual pressure responsive mechanism 49. This mechanism includes abellows 50 or other pressure responsive element disposed in a housing 5|to which a pressure is transmitted by the master regulator. It alsoincludes another bellows 52 disposed within a housing 53 to whichpressure impulses are transmitted from valve III of regulator 9.Whenever a change in the pressure impulse sent out from the masterregulator occurs, the air control regulator I responds irrnnediately toeffect an adjustment in the rate of flow of air through the pulverizer.This impulse from the master regulator also acts immediately on bellows50 associated with regulator 6' causing that regulator to immediatelyrespond and shift the rheostat to a point corresponding to the speedrequired of the coal feed motor 5.- The loading pressure sent out by themaster regulator also acts on bellows II of regulator 9 to effect anunbalance between the turbine steam pressure responsive bellows I2 andbellows II, whereby a change is produced in the magnitude of theimpulses sent out from this regulator to bellows 52 of the feedcontrolling regulator. This change in pressure acts to operate regulator6' and cause it to further increase or decrease, as the case may be, thespeed setting of the rheostat 6. These adjustments continue untilbellows II and I2 of regulator 9 are in balance at which time the ratesof feed of fuel and of air flow to the pulverizer will be such that theproper amount of pulverized fuel is delivered to the furnace and thatthe amount of raw coal fed to the pulverizer will correspond to theamount of pulverized fuel delivered by the pulverizer to the furnace. n

In Fig. 3 of the drawings, I have illustrated regulator 9 in a formmodified to be operable in the case where the pulverizer mill is drivenby an electric motor as indicated at 53. For this purpose a plurality ofcoil windings 55 and 56 have been substituted for bellows I2 ofregulator 9. One of these coils is stationarily mounted and the othermovably mounted. One coil is arranged to be energized in accordance withthe voltage of the power supply for the mill motor 53. The other coil istraversed by the current delivered to the motor or by a current whosevalue is proportional to the current delivered to the motor. These coilsare so disposed with respect to each other that they move towards eachother with increasing load on the motor and away from each other whenthe load on the motor decreases. One coil, for example coil 55, iscarried by a yoke 51 connected to lever I3 of the regulator 9'. Thisyoke is also connected by spring 32 to piston 33 of dash-pot 34described in connection with regulator 9 of Fig. 1. In other respectsthe regulator in Fig. 3 operates in subcontrol system would be as shownand described in connection with Figs. 1 and 2.

Regulators 6 and l, of Figs. 1 and 2, may be of any suitable type. Asillustrated, these are of the pneumatic cylinder type, that is a typehaving a cylinder in which a reciprocating piston is disposed andprovided with a piston rod 59 carrying aframe 60 connected to theelement to be regulated. Movement of the piston in either direction iscontrolled by a pilot valve 6| actuated by a diaphragm to which theimpulses from a sending line 2i of Fig. 1 are transmitted or from thesending line 2| of the master regulator of Fig. 2. A regulator of thistype is shown and described in United States Letters Patent No.2,044,936 to Robert R. Donaldson and dated June 23, 1936. Theseregulators as described in the patent, are compensated in their action,i. e. the piston comes to rest when it has moved a predetermineddistance in either direction depending on the extent to which the pilotvalve is opened, and if, when in such position of rest,

more regulation is required, the piston moves to another position and soon until the proper position is attained. Thus these regulators arecapable of operating step-by-step in either direction, slowly in smallincrements or faster in larger increments-or they are capable of makinga full stroke in either direction in one motion provided the pilot valveis held open by its actuating device because a condition undergoingregulation requires it. Since the operation and construction of theseregulators are fully disclosed in the patent just referred to furtherdetailed description thereof herein is believed unnecessary in that noclaim is made to these regulators per se.

The regulator which adjusts the rheostat of Fig; 2 is similar to theregulators 6 and! and differs therefrom only in the mechanism whichoperates the pilot valve 6| thereof. This pilot valve is operated bybellows 50 and 52 each of which is provided with a push rod 63 to whicha floating beam 64 is pivotally connected at its opposite ends. At somepoint between the connection of this floating beam to the push rods, a

link 65 is connected and this link is in turn connected to a lever 66that operates the pilot valve 6|. -If either one or both of thesebellows are subjected to an increasing pressure, the push rods thereofmove upwardly causing the regulator to move in one direction to adjustthejrheostat.

The direction of movement in this case would be such as to decrease thespeed of the motor as a reduction in. the rate of supply of fuel isrequired.

forces exerted by bellows l l and spring l8 and bellows l2 on lever I3are in equilibrium. If now the loading pressure imposed by masterregulator M. R. on bellows I I of regulator 9, is changed, regulator 9is unbalanced causing regulators 6' and 1 to make a change in the rateof feed of coal to the pulverizer and in the rate of air supply thereto.The pressure imposed by the pressure of the steam at the nozzles of theturbine on bellows l2 then changes in a direction to place spring l8 andbellows II and I2 in equilibrium. Likewise, a. change in load on theturbine results in a change in pressure on bellows I2 which unbalancesthe regulator to effect a change in feed of coal and the air supplywhereby the steam pressure on bellows I2 is changed until it is inbalance with the loading pressure on bellows II.

In Fig. 2, the operation is somewhat the same, except that the loadingpressure sent out by master regulator acts directly on regulators 6' and1 as well as on bellows ll of regulator 9. Regulator 9 acts only onregulator 5 but the regulating position of regulator l is finallydetermined by a state of balance being maintained between the loadingpressure and the steam pressure of the turbine nozzles actingrespectively on bellows H and I2. And since this loading pressure alsoacts on regulator 1, the required amount of air is supplied to thepulverizer and a balance is maintained between the amount of coaldelivered to the pulverizer and the amount of pulverized coal deliveredto the furnaces for a given demand therefor.

The regulator of Fig. 3 operates on the same principle as the regulator9 of Figs. 1 and 2 operates, but differs therefrom in that the powerdelivered to the motor is measured by coils 55 and 56 and utilized tobalance spring l8 for the various loading pressures on bellows ll, anyunbalancebetween them resulting in a change in feed of coal to thepulverizer and in the rate of air supply thereto until a balance isrestored between them.

Having thus described the invention, what I desire to claim and secureby Letters Patent is:

1. A control system for regulating the output of a pulverizer milldisposed to deliver pulverized fuel to a furnace, and provided with aprime mover for driving the mill and a feeder for deliveringunpulverized fuel to the mill, said control system comprising aregulator responsive to a condition reflecting the demand for pulverizedfuel for causing the feeder to deliver unpulverized fuel to the mill at-a rate approximating the demand for pulverized fuel, and meansresponsive to the load imposed on the prime mover for so modifying theaction of said regulator that the feeder is caused to deliverunpulverized fuel to the mill at substantially the same rate that pul-'verlzed fuel is delivered by said mill to the furnace.

2. A regulator for controlling the operation of a mill for pulverizingcoal, said mill being driven by a prime mover'and provided with a feederfor delivering coal to the mill and a regulator for controlling saidfeeder, said regulator having a control element for governing theoperation of the feeder regulator, an actuating element for sponsivetothe load on the prime mover disposed to be normally in balance withsaid actuating element, said load responsive and actuating ele-- mentsmutually cooperating with said control element to so adjust the rate offeed that a balance is maintained between them.

. 3. A system for controlling the operation of a pulverizer mill drivenby a prime mover and provided with a feeder and regulator for adjustingthe rate of delivery of material by the feeder to the mill, said systemincluding a' regulator having a control mechanism for controlling thefeeder regulator, a spring yieldingly urging said mechanisminonedirection, means for variably loading said spring in accordancewith the demand for p l erizedmaterial to cause said control mechanismto function and change the rate of feed of material to the mill, andmeans responsive to the load on said prime mover and acting inopposition to the pull of said spring on said control mechanism toeffect such a modification of the rate of feed that a balance isestablished between the load responsive means and the loading imposed onthe spring by said loading means.

4. A control system for pulverizer mills driven by a prime mover andprovided with a motor driven feeder and a source of supply of air forcarryin pulverized material out of the mill to a boiler furnace, saidsystem comprising a master regulator responsive to the demand for steamand means under the control of said master regulator for regulating theair supply. and a second regulator having means responsive to theoperation of the master regulator for adjusting the speed of the feedermotor to establish a rate of feed required by the demand for pulverizedmaterial, and means responsive to the load on' the prime mover andcooperating with said regulator responsive means to so modify the speedof the feeder motor that a balance is maintained between the output ofthe mill and the rate of feed of unpulverized material to the mill.

5. Asystm according to claim '7 characterized by the fact that themaster regulator embodies means for establishing control impulses whosemagnitudes are dependent upon the demand for pulverized material andthat said second regulator includes a control element for adjusting thespeed of the feeder motor, an actuating element including a spring foractuating said control element, means responsive to the master regulatorimpulse for adjusting said spring to cause the control'element toregulate thefeeder motor in accordance with the demand for pulverizedmaterial as indicated by the magnitude of the master regulator impulse,and means responsive to the load on the prime mover and acting inopposition to said spring arranged to so modify the action of saidcontrol element that a balance is maintained between the output of themill and the rate of feed thereto for any rate of demand as indicated bysaid master'regulator.

6. A system according to claim '7 characterized by the fact that saidsecond regulator-comprises a regulating element for controlling saidfeeder motor, a spring operatively connected to said regulating elementand tending to move the same in one direction, means under the controlof the master regulator for applying a variable loading force to saidspring, and means actuated in accordance with the load on the primemover for balancing the loading force on said spring, whereby when anunbalance occurs the regulating element is actuated to modify the speedof the feeder motor until the spring loading is again balanced.

7. A control system for pulverizer mills adapted to supply powdered fuelto boiler furnaces, and provided with an adjustable speed motor fordriving the mill, means for feeding unpulverized fuel to the mill atcontrollable rates, and an adjustable supply of air for carryingpowdered fuel out of the mill to the furnace, said system comprising amaster regulator responsive to steam demand for developing a firstcontrol force that is a measure of steam demand, a second regulatorresponsive to said first control force and to a condition that varieswith the load on the mo tor for establishing a second control forcewhose magnitude varies with the value of said first mentioned controlforce and the extent of response of said second regulator to the load onthe motor, and means responsive to said second cont-rol force for socontrolling the unpulverized fuel feeding means that said regulator ismaintained substantially in a state of balance between said firstcontrol force and the effect of the motor load thereon.

8. The combination with a pulverizer mill, a steam actuated prime moverfor driving the mill, and means for feeding material to be pulverized tothe mill at regulatable rates, of means for developing a control forcethat varies with a condition reflecting the demand for pulverizedmaterial, means responsiveto said control force for controlling theoperation of the feeding means to effect different rates of feed to themill in ac cordance with the demand for pulverized material, and meansresponsive to the load on the steam actuated prime mover for somodifying the operation of the feed. controlling means that asubstantially balanced relationship is maintained between the rate offeed to the mill and the load on the prime mover for each rate of feedto the mill.

9. The combination with a pulverizer mill, an electric motor for drivingthe mill, and means for feeding material to be pulverized to the mill atregulatable rates, of means for developing-a control force that varieswith a condition reflecting the demand for pulverized material, meansresponsive to said control force for controlling the operation of thefeeding means to effect different rates of feed to the mill inaccordance with the demand for pulverizedmaterial, and means responsiveto the load on the electric motor for so modifying the operation of thefeed controlling means that a substantially balanced relationship ismaintained between the rate of feed to the mill and the load' on theprime mover for each rate of -feed to the mill.

moms A. PEEBLES.

